This Is Us devoted an entire hour to Jack's early life on Tuesday, in a deeply harrowing and emotional episode that centered exclusively on the years leading up to and during the Vietnam War. Because much of the audience's knowledge and understanding of the Pearsons' beloved patriarch stems from the night he first heard Rebecca sing "Moonshadow" in the bar in 1972, this episode -- simply titled "Vietnam" -- peels back the layers on how his imperfect home life, his relationship with his rarely-mentioned younger brother Nicky and his traumatizing experiences in the war shaped his post-Vietnam persona.

Much of the episode sets up what's to come in regards to the mystery of what happened to Nicky. We know that he died in the war, but we don't know how. We know he was drafted through the televised lottery; his birthday, Oct. 18, was fifth to be picked. We know why Jack, who had a tachycardia (which better explains the fatal heart attack he'd suffer decades later), couldn't originally enlist. That he had plans for Nicky to flee to Canada to escape the war, but that Nicky wanted to be the one to save the day for a change. That an ominous letter Nicky sent home from Vietnam, where he foreshadowed his own death, prompted Jack to go overseas anyway, so he could be his brother's protector.

Their reunion at the end of the episode in Vietnam left much to be desired. Gone was the tight-knit bond the two brothers had throughout their childhood and young adult lives. Instead, there was an uneasy tension and distance between the siblings as they saw each other in the flesh for the first time in a while. With the clock ticking on Nicky's mortality, This Is Us creator Dan Fogelman promised the journey to discovering what led to Jack's brother's death during the war will be both poignant and significant.

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"It’s a big story. It’s surprising. It’s fair to say that clearly, something broke in Vietnam for Jack as it relates to his brother, and I think people will be satisfied with what we’re doing there even if they don’t always like it," Fogelman told ET following a screening of Tuesday's episode. "It takes place in a compressed period of time that was a defining period of time for Jack as it relates to his brother. I think it would be borderline impossible to guess where it’s going because it’s very specific.”

Fogelman revealed that the time in which Jack and Nicky spend time together in Vietnam will be more than 24 hours, and that when this specific storyline picks back up again in a few weeks on This Is Us, it will "be a direct pickup from where this episode ended" of the brothers' unexpected reunion and "move forward through Vietnam linearly."

"It's a couple of weeks to a month of time that was really a defining chunk of time that really created the stuff that Jack keeps at bay in the Jack that we've known throughout the years," Fogelman teased, adding that whatever events led up to Nicky's death will be devastating to watch. "I think it's fair to say that clearly, something traumatic happened. It's not going to suddenly be not that."

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But is it comparable to the traumatic house fire that served as the precursor to Jack's death? Fogelman played coy, but confirmed that the moment in which Nicky dies is just as big -- maybe for different reasons. “It’s comparable to some of our biggest stuff in that it’s going to feel really massive, in terms of what it does to Jack," he explained. "Jack buried a portion of his life when he came back from the war. What we’re going to explore this season is what he buried.”

Though the specifics will be revealed in due time, Fogelman and Michael Angarano, making his anticipated debut as Nicky in this episode, explained why they view the character as a tragic hero. As they told it, it's Nicky's personal motivation to try and stand on his own two feet in an attempt to emerge from the shadow of his brother, Jack.

“Watching how he went to the war and why, I felt pained for him," Fogelman said. "This sweet, sensitive, kind boy who wasn’t meant to do this war but did it in an attempt to step out of his big brother’s shadow and in an attempt to please a father who will never be pleased by it... It’s sad, knowing that he has a tragic story there. Watching how he got over there, by the whim of a birthday, by these external forces that were not fair, it is a tragic story.”

“There’s one thing that you can really see in this episode that really weighs on Nicky: that he is Jack’s brother," Angarano told ET. "He feels the weight of being the younger brother, always being protected, always being looked out for. It’s Jack’s plan. There’s that classic sibling dynamic in which the younger brother’s like, 'Let me go. I can do this. I can be my own person.'"

"It’s also that codependent relationship that he has because he needs it. There’s a part of him that doesn’t know if he can stand on his own without his brother’s help. There’s also the fact that he’s used to his father being inconsistent and negligent and abusive. There’s going to be that effect on his personality that he’s not even aware of. The fact that he’s chain-smoking and he’s at the draft and needs to take shots and drink... He doesn’t understand these things about himself yet."

Angarano zeroed in on the goodbye letter Nicky writes to Jack, expressing his intentions to report for war duty instead of being a draft dodger, as an encapsulation of who the character was to him. "That letter and the choice to go [to war] and to not escape to Canada was a very, very personal, intuitive decision on Nicky’s part -- that he needed to do something for himself," Angarano said. "The line between right or wrong was so blurry but it was something that he needed as Jack’s brother.”

“His heart and his brain are his own worst enemy," he continued. "Take that for what it is, but there’s a part of you that just wants to be like, ‘Just get out of there. Just stop.’ But there’s something in Nicky… you can see how he has to physically internalize everything. He’s so fragile but also incredibly strong.”

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For Milo Ventimiglia, he had nothing but praise for Angarano as the new This Is Us cast member kicks off his arc, saying it's been "amazing" watching his scene partner "embrace the work and everything that the writers are setting up for he and I." Though This Is Us is just scratching the surface on the Vietnam portion of the story, there is much more to come; currently, they're knee-deep in filming the fourth episode featuring a strong Vietnam presence. Select members of the cast and crew -- including Ventimiglia, Angarano and Justin Hartley, whose present-day storyline tees up Kevin to sojourn overseas to discover more about Jack's wartime experience -- are gearing up to fly to Saigon to film on location this week.

“I imagine [the episode is] going to spark conversation. My dad and I, we’ve spoken quite a bit at length about his experience in Vietnam so I’ve already had those conversations. I don’t know what this brings up. I guess we’ll have to wait over the next several weeks to uncover that," Ventimiglia told ET of his father, who is a Vietnam War vet. "I do know that when we went to the D.C. Memorial, I went to the [Vietnam] Traveling [Memorial] Wall with my father and I saw his friends' names on the wall. That was heavy. Knowing that he’s had this whole life, I can’t even begin to imagine what his experiences have been like. I’m sure grateful he made it.”

As for what lies ahead for viewers following Jack and Nicky's tense reunion at the end of the episode, just know it's well worth the wait.

“I’m excited for the audience to see that in a tough upbringing, both Jack and Nicky really leaned on each other and looked out for one another. As much as Jack is the protector, there was something that Nicky was always giving him, which was support and love and kindness when maybe he wasn’t quite getting that," Ventimiglia said. "That’s all Jack really is looking for in life, and it always feels like he just wants to be a part of a loving family. He had that with his brother.”

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“It’s so complex, as all sibling relationships are. There’s so much love. There’s so much resentment, especially from Nicky as his little brother," Angarano said. "There’s respect as grown men that they got through this childhood together and a knowingness that they got through this together and the knowingness that they’re not the same person. That becomes much more abundantly clear as the episodes unfold that they come from the same place but they’re very different. How they handle everything and how they handle the war is very much different.”

When it came to casting the role of Nicky, a character that was first mentioned in the season two episode, "Brothers," Fogelman was looking to differentiate the younger Pearson from Jack in personality, look and demeanor. Angarano, who most recently starred in Showtime's two-season dramedy I'm Dying Up Here, offered exactly what Fogelman was looking for.

“I didn’t want the character to just be a weakling, to be the polar opposite of Milo," Fogelman explained. "I wanted him to be handsome and likable and charming, but in a slightly different way. There’s a sensitivity Michael has. There’s a kindness in his eyes. There’s something he does when he’s acting, even with those big glasses, you’re seeing into this soul.”

Angarano confessed he wasn't aware of the rich backstory already provided to him from the show's second season; it was his sister who broke the news to him that Nicky was dead when he told her he was joining This Is Us. Playing a character as deeply connected to Jack comes with a level of expectation and heft, but Angarano said he didn't feel the pressure because it was all on paper. "Reading the first script, there wasn't anything hanging over. There was no expectation. The scenes were so clear. It was so palpable the sense of dread that Nicky was going through and how he was just like so many other guys on draft day," he said. "At the heart of it, and influenced by that dynamic of his relationship with Jack and his father, it was a really clear character to me. I never felt like any sort of expectation to live up to.”

"We’ve been talking about [Jack's] brother as early as season two, it’s always been that that’s the formative figure in his life. The great surprise was getting Michael and see it come to life and go, ‘Wow, our world just expanded because I care about Jack’s brother.’ You’re feeling it more and more with episodes that you haven’t seen yet," Fogelman noted. "When it works like that and you go, ‘This is going to really work,' that’s always very surprising and exciting.”